Abstract:

Objective In order to explore the influence of the twist direction and twist configuration of the double wrapping on the structure and properties of the double covered yarns, the influences of twist and twist direction arrangement on the structure, tensile fracture performance and twist balance of covered yarns were experimentally investigated and analyzed, which were produced using the hollow spindle method with double wrapping.

Method Three 111 dtex (48 f) untwisted polyester draw texturing yarn (DTY) filament yarns with different colors were employed as raw materials with unchanged wrapping twist, double covered yarns with SZ cross-wrapped and SS co-wrapped were spun separately each with 6 twist ratios, using hollow spindle covering machine. The covering structures of the covered yarns were analyzed by visual observation of the partial enlarged photographs of covered yarns, while the twist balance indexes were tested by open loop method and the tensile mechinical properties were tested by yarn tensile tester.

Results The spinning principle of double covered yarns was explained, and the structural model of the two twist direction arrangement of the same direction wrapping and the reverse (cross) wrapping were establi-shed (Fig. 1 and 2). Partial enlarged views of two types of double covered yarns showed that both types of covered yarns presented mixed color of core yarn and wrapped yarn, and it was found that with the increase of secondary wrapped twist, the finer the degree of color block segmentation mixed and the finer the texture of covered yarn. Meanwhile, the cross-wrapped was found to be able to form a clear layered wrapping structure. Double covered yarns and raw filament yarn showed similar force-extension curves, and the differences were only in the characteristic values like yield load and yield elongation, breaking strength and elongation at break, based on 50 core yarn tests. SZ cross-wrapped and SS co-wrapped covered yarns were produced with twist ratio of 0.9. The tensile test results of three types of raw yarn and two types of 12 covered yarns presented the changes of breaking strengths and their CV value, elongations at break and their CV value and strength efficiencies along with the changes of twist direction and twist ratio of twice wrapping (Tab. 1). Different twist direction and twist ratio arrangement produced different twist balance in the double covered yarn, the snarl index test results of 12 composite yarns showed the self-twisting direction, mean values of snarl indexes and their CV value for 10 tests of the two types of double covered yarns with different twist directions and twist ratios (Tab. 2).

Conclusion It was discovered that co-wrapped can not form a layered wrapping structure similar to cross-wrapped, and the strength efficiencies of the two types of covered yarns are greater than 1. The elongations at break are increased by more than 25%, and the strength of covered yarns increase first and then decrease with the increase of twist ratio, but the change range are small. The residual torques of the twice wrappings are not a simple superimposition. The second wrapping has a consolidation effect on the residual torque of the primary wrapping, and the snarl index of co-wrapped covered yarn is much larger than that of cross-wrapped. Reasonable configurating of the twist ratio of the twice wrapping can significantly reduce the snarl index of the cross-wrapped covered yarn. This paper would provide reference for the process design of double wrapped covered yarn.

Key words: twist, twist direction, double covered yarn, tensile fracture property, twist balance, residual torque, snarl index